[Bug] Shell top bar inconsistent color

[TimLandskron]

Description

In the attached reproduction project, there's a Xamarin.Forms Shell application created with the VS for mac template. I adjusted the "Browse" Tab to contain 2 subtabs so we get a top tab bar. The ItemsPage has two Shell properties set like this:
Shell.TabBarBackgroundColor="Red"
Shell.BackgroundColor="Red"
The ItemDetailPage (reached when clicking one of the items) does not set any Shell properties. Now when we click an item to show the ItemDetailPage in Modal presentation style, and immediately change the tab from the top tab bar, we see that Shell's top bar is changing its background color to the default blue color of xamarin. This behavior is attached as a gif because it's hard to reproduce, there's only a small time frame to change the tab.

Steps to Reproduce

1. Download and unzip the attached project.
2. Build and deploy.
3. Switch to the "Tabs" tab from the bottom tab bar.
4. Tap an item
5. Switch the tab before the item detail page is pushed
6. Close the item detail page

Expected Behavior

The Shells top bar should stay red and not switch to the default blue color.

Actual Behavior

The Shells top bar is changing to the default blue color.

Basic Information

• Version with issue: 5.0.0.2012
• Last known good version: -
• Platform Target Frameworks:
  • Android: Android 11.0 , API 30

Environment

Show/Hide Visual Studio info

=== Visual Studio Enterprise 2019 for Mac ===

Version 8.9.10 (build 4)
Installation UUID: 5e901ac2-3e3c-45fd-8a27-91f1d8f80b93
	GTK+ 2.24.23 (Raleigh theme)
	Xamarin.Mac 6.18.0.23 (d16-6 / 088c73638)

	Package version: 612000125

=== Mono Framework MDK ===

Runtime:
	Mono 6.12.0.125 (2020-02/8c552e98bd6) (64-bit)
	Package version: 612000125

=== Roslyn (Language Service) ===

3.9.0-6.21152.10+c10f884b30737542ddd84ca889a4aad9281ce210

=== NuGet ===

Version: 5.8.0.6860

=== .NET Core SDK ===

SDK: /usr/local/share/dotnet/sdk/5.0.203/Sdks
SDK Versions:
	5.0.203
	5.0.202
	5.0.201
	3.1.409
	3.1.408
	3.1.407
MSBuild SDKs: /Applications/Visual Studio.app/Contents/Resources/lib/monodevelop/bin/MSBuild/Current/bin/Sdks

=== .NET Core Runtime ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
	5.0.6
	5.0.5
	5.0.4
	3.1.15
	3.1.14
	3.1.13

=== .NET Core 3.1 SDK ===

SDK: 3.1.409

=== Xamarin.Profiler ===

Version: 1.6.15.68
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Updater ===

Version: 11

=== Xamarin Designer ===

Version: 16.9.0.324
Hash: b1e216c75
Branch: remotes/origin/d16-9
Build date: 2021-04-16 00:02:50 UTC

=== Apple Developer Tools ===

Xcode 12.5 (18205)
Build 12E262

=== Xamarin.Mac ===

Xamarin.Mac not installed. Can't find /Library/Frameworks/Xamarin.Mac.framework/Versions/Current/Version.

=== Xamarin.iOS ===

Version: 14.16.0.5 (Visual Studio Enterprise)
Hash: ab40b131d
Branch: xcode12.5
Build date: 2021-04-22 08:56:42-0400

=== Xamarin.Android ===

Version: 11.2.2.1 (Visual Studio Enterprise)
Commit: xamarin-android/d16-9/877f572
Android SDK: /Users/timlandskron/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		None installed

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 30.0.4
SDK Build Tools Version: 30.0.2

Build Information: 
Mono: 5e9cb6d
Java.Interop: xamarin/java.interop/d16-9@54f8c24
ProGuard: Guardsquare/proguard/v7.0.1@912d149
SQLite: xamarin/sqlite/3.34.1@daff8f4
Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-9@d210f11

=== Microsoft OpenJDK for Mobile ===

Java SDK: /Users/timlandskron/Library/Developer/Xamarin/jdk/microsoft_dist_openjdk_1.8.0.25
1.8.0-25
Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Android SDK Manager ===

Version: 16.9.0.22
Hash: a391de2
Branch: remotes/origin/dev/bretjohn/readme-tweak~3
Build date: 2021-05-13 17:03:56 UTC

=== Android Device Manager ===

Version: 16.9.0.17
Hash: fc2b3db
Branch: remotes/origin/dev/jmt/d16-9bump~1
Build date: 2021-05-13 17:04:12 UTC

=== Build Information ===

Release ID: 809100004
Git revision: 30612572b7defb0273729cb6b5ea5adc8bd9a147
Build date: 2021-05-13 13:06:24-04
Build branch: release-8.9
Xamarin extensions: 30612572b7defb0273729cb6b5ea5adc8bd9a147

=== Operating System ===

Mac OS X 10.16.0
Darwin 20.4.0 Darwin Kernel Version 20.4.0
    Thu Apr 22 21:46:47 PDT 2021
    root:xnu-7195.101.2~1/RELEASE_X86_64 x86_64

Screenshots

ShellTopBarIssue.mov

Reproduction Link

ShellTopBarIssue.zip

Workaround

One possible solution:
Every content page which is pushed modally needs to set the Shell.BackgroundColor & Shell.TabBarBackgroundColor properties.